Tunable microwave time delay equalizer



Sept. 9, 1969 T. A. ABELE ETAL TUNABLE MICROWAVE TIME DELAY EQUALIZER 2Sheets-Sheet 1 Filed Dec. 29. 1966 T. A. ABELE uws/vngis. WANG Sept. 9,1969 Filed Dec. 29. 1966 FIG. 2

T. A. ABELE ETAL TUNABLE MICROWAVE TIME DELAY EQUALIZER 2 Sheets-Sheet zFIG. 3

RELATIVE TIME DELAY NANOSECONDS FREQUENCY-MHZ United States Patent Int.Cl. H03h 5/00 US. Cl. 33328 6 Claims ABSTRACT OF THE DISCLOSURE A timedelay equalizer of the type in which a cylindrical cavity simultaneouslyresonant in two orthogonal modes is coupled to a rectangular waveguide.Specifically, the coupling comprises a relatively large transverselymovable aperture in a common wall between the end of the cavity and onewide wall of the guide and a relatively large probe extending anadjustable height in the guide from the other wide wall at a pointbetween the axis of the aperture and a narrow wall. Simultaneousadjustment of the aperture position and the height of the probe variesthe shape of the time delay vs. frequency characteristics introduced bythe cavity without producing an impedance discontinuity.

Background of the invention The invention relates to a time delayequalizer for electromagnetic wave networks and more particularly to anequalizer of the type known to the art as the dual mode cavityresonator.

As previously taught by L. C. Tillotson in Patent 2,795,763, grantedJune 11, 1957 and more recently by S. B. Cohn in Patent 3,277,403,granted Oct. 4, 1966, a dual mode resonator coupled to a waveguide andconnected to a microwave network can equalized typical distortion in theshape of the time delay vs. frequency characteristic of electromagneticwave energy traveling through the network provided the two modes of thecavity are at the same resonant frequency, are of equal amplitude and inphase quadrature with each other, introduce a transmission coefiicientthat complements the time delay vs. frequency characteristic of thenetwork, and do not introduce any mismatch.

As simple and as useful as such a component appears to be, it has beenfound in practice that it is impossible to precisely estimate in advancethe time delay vs. frequency characteristic to be equalized and then tospecifically design a cavity and its coupling mechanism to thischaracteristic. In other words some adjustment of each equalizer foreach application is essential. This requires a continuous adjustabilityof the loaded Q of the cavity without introducing losses due toreflections at the aperture. No method suitable for this purpose isknown in the art.

Summary of invention In accordance with the present invention aparticular coupling means between a rectangular waveguide and acylindrical equalizing cavity has been developed having pluraladjustable parameters which when made simultaneously in proper sensescan shape the time delay vs. frequency characteristic introduced and atthe same time maintain reflection losses at a minimum. In particular thecoupling means comprises a large aperture that can be moved transverselyacross the wide dimension of the guide and a large conductive probe ofadjustable height located between the axis which extends through thecenter of the aperture and the narrow wall of the guide. According to apreferred embodiment, the aperture is a square 3,466,573 Patented Sept.9, 1969 "ice approximately one-quarter wavelength on a side and theprobe is a cylinder approximately one-half of this dimension indiameter. The specific dimension of the aperture can vary fromapproximately one-eight to threeeighth wavelength depending upon thetime delay vs. frequency characteristic to be equalized. Large aperturesproduce broader characteristics. Contrasting with a probe of thisdimension, the small, nonresonant probe used by the prior art tointroduce a reactance for impedance matching would not be expected tochange the time delay vs. frequency characteristic of a cavity resonatorequalizer. In the structure according to the invention the large sizeand asymmetrical location of the probe has been found to increase thecoupling coefficient between the guide and the cavity and in turn tobroaden the time delay vs. frequency characteristic when the probelength is increased. Simultaneous transverse movement of the aperturemaintains the reflection loss at a minimum.

Brief description of drawing FIG. 1 is a perspective view of a cavityequalizer in accordance with the invention;

FIG. 2 is a cross-sectional view of FIG. 1 indicating typical dimensionsfor a given embodiment; and

FIG. 3 shows typical time delay vs. frequency characteristics forseveral adjustments of the structure of FIG. 1.

Description of illustrative embodiment Referring more particularly toFIG. 1, an illustrative embodiment of an equalizer in accordance withthe invention is shown comprising a short section of conductivelybounded rectangular waveguide 11 which can be connected to theconventional flanges of connecting waveguides of the network to beequalized. Guide 11 is formed of a first casting comprising a channelincluding one wide wall 12 integrally formed With narrow walls 13 and 14and a separate top wall 15 suitably bolted by screws 17 to walls 13 and14 through elongated holes 16 so that wall 15 may be moved transversely(perpendicular to narrow walls 13 and 14 and to the longitudinal axis ofguide 11). A square aperture 18 is located in wall 15 in such positionthat transverse movement of wall 15 carries aperture 18 between oneposition in which one edge thereof is substantially adjacent to wall 13and in another in which the opposite edge thereof substantiallycoincides with the center plane of guide 11. A large threaded probe 19extends through a threaded hole in wall 12. Centered above aperture 18is a closed cylindrical cavity 20 which as illustrated has a top wall 21and utilizes a reduced thickness portion of wall 15 of guide 11 as thebottom wall of the cavity. It should be understood, of course, thatother physical constructions can be utilized.

The distance between the bottom wall 15 and the top wall 21 of cavity 20determines its basic resonant frequency and should in general beone-half wavelength, or a multiple thereof, at the center of the band tobe equalized. Tuning screws 22 and 23, respectively aligned with thetransverse and longitudinal dimensions of guide 11, provide fine tuningof the resonant frequency in accordance with well-known cavity tuningprinciples.

Since the relative sizes of the components described and their locationsare important with respect to the invention, dimensions of a typicalembodiment are illustrated on the cross-sectional view of FIG. 2. TheWaveguide channel has the conventional internal dimensions of 1.145" x2.290" and the embodiment is designed to equalize around a midbandfrequency of 3930 mHz. (a guide wavelength of 3.98 inches in guide 11).An embodiment designed to operate at another frequency would beproportionaly scaled. In this connection, several generalizations can bemade in terms of Ag, the guide Wavelength at a given midband frequencyin guide 11. For the particular time delay vs. frequency characteristicsshown in FIG. 3 aperture 18 should be in the order of xg/4 on each sideand should have an average position x measured from the nearer narrowwall 13 to the center of aperture 18 of Ag/ 6 or \g/7. Probe 19 shouldhave a diameter in the order of one half the dimension of aperture 18 orAg/ 8 and should be located as close to the nearer narrow wall 13 as isphysically practical. The average penetration y of probe 19 is slightlygreater than one half its diameter. For wider or narrower time delay vs.frequency characteristics than those shown in FIG. 3 the dimensions ofthe aperture can be made between Fug/8 and Ag/ 8 respectively on eachside and all other dimensions will have to be varied accordingly.

With these proportions, the structure of FIG. 1 can be used toneutralize the delay distortion introduced, for example, by one or moremicrowave bandpass filters of the type which typically introduce a timedelay vs. frequency distortion that is minimum at the center of the bandand increases on either side thereof to the filter cutoff frequency. Toequalize such a characteristic the invention introduces a time delay vs.frequency characteristic of adjustable shape that varies in the oppositemanner, that is, a characteristic that is maximum at the center of theband and decreases on either side thereof. It is critically importantthat the cavity does not introduce reflection losses or dissipationlosses.

Operation of the invention depends upon the fact that the transverse andlongitudinal magnetic field components of the wave propagating in onedirection along guide 11 couple respectively to separate cross-polarizedmodes in cavity 20 through aperture 18. These modes are in phasequadrature to each other since the transversal and longitudinal magneticfield components in waveguide 11 are in phase quadrature. In additionboth modes are of equal amplitude since aperture 18 is located in aregion where the magnetic field components in the waveguide 11 are ofequal amplitude. Each mode carries energy back into guide 11 and sincethey are equal in amplitude their relative phases cause them to cancelwith each other in guide 11 at every frequency for propagation in thebackward direction, that is, for travel toward the input. In the forwarddirection, they add and introduce a time delay that is a function offrequency. It has been recognized that the precise time delay vs.frequency shape of the transmission coefiicient depends upon thecoupling coefiicient between the modes in cavity 20 and fields withinguide 11. While this coupling coefiicient could be varied by varying thesize of aperture 18, the mechanical problems so involved would producean impractical structure. In accordance with the invention it has beenfound that increasing the penetration of probe 19. of the size and inthe location described, accompanied by a movement of aperture 18 towardthe center of guide 11, increases the coupling coefiicient withoutunbalancing the two cavity modes. Ordinarily increasing the penetrationof a reactive probe merely increases the reactance which it introducesin the waveguide. The unexpected performance observed for the inventioncan be explained in a qualitative Way, which may not be rigorous from amathematical standpoint, by recognizing that the relatively large sizeof both aperture 18 and probe 19 produces substantial local perturbationof the magnetic field components in guide 11. Increasing the penetrationof probe 19 appears to increase the field concentration near top wall 15at the location of aperture 18 in a manner analogous to reducing theheight of the guide. However, the longitudinal component of the wavefield is increased more than is the transverse component. Thereforesimultaneous transverse movement of aperture 18 equalizes the amplitudeof the modes. While applicant does not wish to be bound by the foregoingexplanation, the experimental data illustrated in FIG. 3 appears toconfirm it. Thus curves A, B, and C respectively, of FIG. 3 are plots ofthe time delay in nanoseconds vs. frequency in mHz. for three indicatedinsertions g of screw 19 in a structure otherwise having dimensionsshown in FIG. 2. In each case the indicated position x of aperture 18retains a return loss in excess of 40 db over a band greater than 10times that of the equalizer band itself. A selection from curves A, B,and C or from a curve of intermediate shape can precisely match theshape of the time delay distortion to be equalized.

While the shape of aperture 18 has been illustrated as square and thecross section of probe 19 as circular, it should be understood thatother shapes for each may be employed provided their relative dimensionsremain substantially the same as those described. Thus, aperture 18 maybe circular and probe 19 square or both components may have the sameshape.

In all cases it is to be understood that the abovedescribed arrangementsare merely illustrative of a small number of the many possibleapplications of the principles of the invention. Numerous and variedother arrangements in accordance with these principles may readily bedevised by those skilled in the art without departing from the spiritand scope of the invention.

We claim:

1. A time delay equalizer for introducing a shaped time delay vs.frequency characteristic to electromagnetic wave energy of the type inwhich a length of conductively bounded rectangular waveguide having apair of wide walls and a pair of narrow walls is coupled to orthogonalmodes in a cavity structure characterized in that said guide and saidcavity are coupled by an aperture on one Wide wall of said guide and aprobe extends from the other wide wall in opposition to said aperture ata point between the aperture center and the nearer narrow wall of saidguide.

2. The equalizer according to claim 1 wherein said aperture isadjustable in transverse position on the wide wall of said guide andsaid probe is adjustable in its extent into said guide.

3. The equalizer according to claim 1 wherein said aperture has aminimum cross-sectional dimension of at least one-eighth of the guidewavelength of wave energy in the band to be equalized.

4. The equalizer according to claim 1 wherein said aperture has aminimum cross-sectional dimension of substantially one-quarter of theguide wavelength of wave energy in the band to be equalized.

5. The equalizer according to claim 3 wherein said probe has a minimumcross-sectional dimension at least half said cross-sectional dimensionof said aperture.

6. The equalizer according to claim 3 wherein said aperture is squareand said probe is a circular cylinder.

References Cited UNITED STATES PATENTS 3,277,403 9/1966 Cohn 333-28HERMAN KARL SAALBACH, Primary Examiner T. VEZEAU, Assistant Examiner US.Cl. X.R. 33333, 83, 98

